1. Field of the Invention
The present invention generally relates to a method for fabricating a bipolar transistor in an integrated circuit (IC), and more particularly, it relates to an improvement in a method for fabricating electrode contact portions of the bipolar transistor.
2. Description of the Prior Art
In general, a bipolar transistor in an IC is formed in an island which is isolated electrically by a method such as p-n junction isolation, oxide film isolation or triple diffusion. Such a semiconductor device is disclosed in, e.g., U.S. Pat. No. 4,445,268, and formation of a self-aligned semiconductor device is disclosed in "Subnanosecond Self-Aligned I.sup.2 L/MTL Circuits", IEEE Transactions on Electron Device, Vol. ED-27, No. 8, August 1980, P. 1379. Further, silicides for IC applications are described by T. Hirao et al. in Extended Abstracts of the 17th Conference on Solid State Devices and Materials, Tokyo, 1985, pp. 381-384 and by S. P. Murarka in SILICIDES FOR VLSI APPLICATIONS, pp. 66-69, 1983, Academic Press.
FIGS. 1A to 1E are cross-sectional views showing the principal steps of a conventional method of fabricating a bipolar transistor in an IC. The conventional method is now described with reference to these drawings. An n.sup.+ -type layer 2 of high impurity concentration for implementing a buried collector layer is selectively formed on a p.sup.- -type silicon substrate 1 of low impurity concentration, followed by growth of an n.sup.- -type epitaxial layer 3 thereover (FIG. 1A).
Then, the substance is selectively oxidized by utilizing a nitride mask film 201 on an under-layer oxide film 101, whereby a thick isolation oxide film 102 is formed while a p-type channel-cut layer 4 is simultaneously formed under the isolation oxide film 102 (FIG. 1B).
The nitride film 201 and the under-layer oxide film 101 are then removed to newly form an oxide film 103 for preventing ion channelling in the silicon crystal during ion implantation by which a p.sup.+ -type layer 5 for implementing an extrinsic base layer at a later stage is formed with a photoresist mask film (this mask film is not shown). Thereafter, the photoresist film is removed to newly form a photoresist mask film 301 with which a p-type layer 6 for implementing an active base layer at a later stage is formed by ion implantation (FIG. 1C).
The photoresist film 301 is then removed and the substance is covered by a passivation film 401 generally made of phospho-silicate glass (PSG). The substance is then subjected to a heat treatment for annealing the ion implanted layers 5 and 6 to form an extrinsic base layer 51 and an active base layer 61 at an intermediate stage as well as densificating the PSG film 401, followed by formation of holes 70 and 80 through both the PSG film 401 and the oxide film 103 to form an n.sup.+ -type layer 7 for implementing an emitter layer and an n.sup.+ -type layer 8 for implementing a low resistance layer underneath a collector electrode by ion implantation (FIG. 1D).
Thereafter, the respective ion implanted layers are annealed to complete an extrinsic base layer 52 and an active base layer 62 and to form an emitter layer 71 and a low resistance layer 81, followed by formation of hole 50 for a base electrode. Then, the respective holes 50, 70 and 80 are provided with films 501 of metal silicide such as platinum silicide (Pt-Si) or palladium silicide (Pd-Si) for preventing junction-spike of the electrodes, followed by formation of a base lead wire 9, an emitter lead wire 10 and a collector lead wire 11 made of a low-resistance metal such as aluminum (Al) (FIG. 1E).
FIG. 2 is a plan view showing a pattern of single-base structure which corresponds to FIG. 1E.
Generally, the frequency characteristic of a bipolar transistor depends on the base-collector capacitance and the base resistance, both of which must be decreased for improving the frequency characteristic. The p.sup.+ -type extrinsic base layer 52 is provided for lowering the base resistance in the aforementioned structure, whereas the provision of the same leads to increase in the base-collector capacitance. In FIG. 2, an inactive base area between the emitter area 71 and an isolation oxide film boundary A also increases the base-collector capacitance. Thus, the emitter area 71 may be bounded by the isolation oxide film to be in walled emitter structure. However, such a method involves various disadvantages as will be seen from FIGS. 3A to 3C.
FIGS. 3A to 3C are partial enlarged sectional views taken along the line X--X in FIG. 2. In FIG. 3A, boron is injected with a photoresist mask film 301 to form a base layer. Then, etching in the walled emitter structure is enhanced at the boundary of an isolation oxide film 102 as indicated by a character A in FIG. 3B, and thus the emitter layer 71 is locally deepened as shown at B in FIG. 3C. Thus, lowered is controllability of the current amplification factor and increased is possibility of emitter-collector short circuit at the point B in FIG. 3C.
Further, as shown in FIG. 2, the base resistance depends on a separation D between the emitter area 71 and the base electrode 501 (hole 50), i.e., the separation between the base wire 9 and the emitter wire 10 plus the total width of margins of the respective wires 9 and 10 extending beyond the respective width of holes 50 and 70, and such margins inevitably remain even if the distance between the lead wires 9 and 10 is reduced by improving accuracy of photoetching.
The transistor may also be brought in the double-base structure as shown in FIG. 4 for reducing the base resistance, as well known in the art. However, the increased base area in the double-base structure results in increase of the base-collector capacitance.
Further, in the conventional fabricating method, the emitter-base interface is formed deeper than the base surface on which the base electrode is formed as seen in FIG. 1E, and this fact causes a problem that the current amplification factor is strongly dependent on the current. Namely, in the range of small current, the current is partly absorbed due to the recombination of electrons and positive holes in the vicinity of the emitter-base interface, and thus the controllability of the current amplification factor is not good.